MDEV-6880 Can't define CURRENT_TIMESTAMP as default value for added column

ALTER TABLE: don't fill default values per row, do it once.
And do it in two places - for copy_data_between_tables() and for online ALTER.

Also, run function_defaults test both for MyISAM and for InnoDB.

mysql-test/t/function_defaults_innodb.test

@@ -0,0 +1,24 @@
+--echo #
+--echo # Test of function defaults for any server, including embedded.
+--echo #
+
+--source include/have_innodb.inc
+set default_storage_engine=innodb;
+
+--echo #
+--echo # Function defaults run 1. No microsecond precision.
+--echo #
+let $current_timestamp=CURRENT_TIMESTAMP;
+let $now=NOW();
+let $timestamp=TIMESTAMP;
+let $datetime=DATETIME;
+source 'include/function_defaults.inc';
+
+--echo #
+--echo # Function defaults run 2. Six digits scale on seconds precision.
+--echo #
+let $current_timestamp=CURRENT_TIMESTAMP(6);
+let $now=NOW(6);
+let $timestamp=TIMESTAMP(6);
+let $datetime=DATETIME(6);
+source 'include/function_defaults.inc';